Abstract
This paper presents results of the laboratory investigations leading to development of a novel, radically different waterflooding process, which is gravity stable. The new process is called Toe-To-Heel Waterflooding (TTHW) and uses vertical wells for injection, while horizontal wells are utilized for production. Normally, the system uses a staggered line drive configuration in which the toe of the horizontal producer is close to the line of vertical injectors. The horizontal section of producer is located at the top of oil formation while the vertical injectors are perforated on the lower section of the layer. By using the proposed configuration, a short-distance oil displacement process is achieved. The water/oil segregation causes the injected water to slump, while the horizontal well as a sink causes water to flow upwards; the composite of these two tendencies leads to an advancement of water through almost all the entire pay section, with the water break-through at the toe, followed by an advancement of the displacement front along and under the horizontal leg.
A 2-D Hele-Shaw model was used for investigations. The Hele-Shaw model was composed of two parallel plexiglass plates, which were held together at 0.1 mm; these vertical plates form between them a simulated porous medium with a permeability of about 833 D. The rectangular chamber forming the simulated porous medium has the dimensions of 52cm* 21.4cm*0.01cm.
The Hele-Shaw model was intended to mimic the vertical section of an oil reservoir and the investigations were designed specifically for a comparative study of vertical sweep efficiency for conventional waterflooding and toe-to-heel waterflooding. These experiments do not permit an evaluation of areal sweep efficiency.
In fact, two toe-to-heel processes were explored: one-phase TTHW and two-phase TTHW; in the last case a blanket of water is intentionally generated at the lower part of layer (by keeping opened the vertical pilot hole of the horizontal producer, while the horizontal leg is closed) and then only the horizontal leg is kept opened (while the vertical pilot hole is closed).
More than 20 low pressure (40-80psi) tests were carried out at room temperature. First, some preliminary experiments were conducted in order to establish a base line performance, to provide calibration, or to study the main mechanisms of this process. In these tests, colored water miscibly displaced plain water in a toe-to-heel configuration; no gravity segregation effect between displacing and displaced liquid existed. Also, no mobility effects and two-phase flow along the horizontal leg existed. Then, detailed experiments with five different oils (with viscosity in the range 10 mPa.s to 12,000 mPa.s), and water injection rates of 2.5 ml/hr to 320 ml/hr were carried out.
A comparison between conventional waterflooding and TTHW showed that in general for the heavy oils with moderate viscosities, oil recoveries at water-break-through are similar, but at the water-out, at the end of the test, the ultimate oil recoveries were much higher for TTHW; this was a direct consequence of higher vertical sweep efficiencies obtained. Later on, 3-D tests conducted in a real porous medium re- inforced that the concept was sound; the 3-D tests results will be reported in part ll of this series.
